Nov 1, 2024 05:13 PM
https://physics.aps.org/articles/v17/142
INTRO: Earth’s inner core is a hot, solid ball—about 20% of Earth’s radius—made of an iron alloy. The planet’s outer core, beneath the rocky mantle, is a colder, liquid metal. Geophysics models explain that, since the movement of a liquid metal induces electrical currents, and currents induce a magnetic field, convection and rotation produce our planet’s magnetic fields. But these models typically neglect an important contribution: how Earth’s magnetic field influences the very flows that generate it.
Alban Pothérat of Coventry University, UK, and collaborators have now developed a theory that accounts for such feedback and vetted it using a lab-based “Little Earth Experiment”. Their results inform a model pinpointing processes that might explain the discrepancies between theoretical predictions and satellite observations of Earth, opening new perspectives on the study of geophysical flows.
Understanding flows in planetary interiors is a long-standing challenge. “If you don’t account for the fact that the magnetic field itself changes the flow, then you won’t get the right flow,” says Pothérat. Indeed, both satellite data from the European Space Agency’s Swarm mission and state-of-the-art numerical simulations indicate certain circulating core flows where liquid produced at the boundary of the inner core is fed into the outer core, moves upward toward the poles, and from there finally flows back inward (Fig. 1).
These observations can’t be explained by the established theory for rotating fluids, which assumes that magnetic-field-induced forces on the flow can be neglected, as they are dominated by rotation-induced Coriolis forces. If rotation is fast enough, goes the theory, flows of liquid are two dimensional and lie in the plane normal to the rotation axis.
For planetary interiors, this imposes a constraint known as the Taylor-Proudman theorem: Fluid cannot flow across a boundary, called the tangent cylinder, defined by a radial distance equal to the solid-core radius.
The flows documented on Earth, however, violate this condition. To explain the discrepancy, what’s needed is an experiment that captures convection, rotation, and magnetism all at once, says Pothérat... (MORE - details)
INTRO: Earth’s inner core is a hot, solid ball—about 20% of Earth’s radius—made of an iron alloy. The planet’s outer core, beneath the rocky mantle, is a colder, liquid metal. Geophysics models explain that, since the movement of a liquid metal induces electrical currents, and currents induce a magnetic field, convection and rotation produce our planet’s magnetic fields. But these models typically neglect an important contribution: how Earth’s magnetic field influences the very flows that generate it.
Alban Pothérat of Coventry University, UK, and collaborators have now developed a theory that accounts for such feedback and vetted it using a lab-based “Little Earth Experiment”. Their results inform a model pinpointing processes that might explain the discrepancies between theoretical predictions and satellite observations of Earth, opening new perspectives on the study of geophysical flows.
Understanding flows in planetary interiors is a long-standing challenge. “If you don’t account for the fact that the magnetic field itself changes the flow, then you won’t get the right flow,” says Pothérat. Indeed, both satellite data from the European Space Agency’s Swarm mission and state-of-the-art numerical simulations indicate certain circulating core flows where liquid produced at the boundary of the inner core is fed into the outer core, moves upward toward the poles, and from there finally flows back inward (Fig. 1).
These observations can’t be explained by the established theory for rotating fluids, which assumes that magnetic-field-induced forces on the flow can be neglected, as they are dominated by rotation-induced Coriolis forces. If rotation is fast enough, goes the theory, flows of liquid are two dimensional and lie in the plane normal to the rotation axis.
For planetary interiors, this imposes a constraint known as the Taylor-Proudman theorem: Fluid cannot flow across a boundary, called the tangent cylinder, defined by a radial distance equal to the solid-core radius.
The flows documented on Earth, however, violate this condition. To explain the discrepancy, what’s needed is an experiment that captures convection, rotation, and magnetism all at once, says Pothérat... (MORE - details)